1. Field of the Invention
This invention relates to the preparation of carbamates, and more specifically to the preparation of carbamates by the direct carbonylation of primary and secondary amines.
2. Description of the Prior Art
Carbamates, which are also referred to herein as "urethanes", are industrial chemicals of enormous significance, and much research has been performed in search of economical processes for their manufacture. One such process involves the formation of a primary amine such as aniline from the corresponding organic nitro compound (e.g., nitrobenzene) and reaction of the resulting primary amine with phosgene to form a carbamyl chloride salt which is then thermally decomposed to the corresponding isocyanate. Recovery and reaction of the isocyanate with an alcohol yeilds the carbamate. Due to the high toxicity of phosgene, and to the corrosive nature of systems in which the chloride ion is employed, alternative processes have been sought which would remove these disadvantages.
Accordingly, a large number of processes were developed to carbonylate an organic nitro compound with carbon monoxide in the presence of an organic hydroxy compound and certain catalyst systems to obtain the corresponding urethane. Exemplary references to these processes and their catalyst systems are as follows: (1) U.S. Pat. Nos. 3,338,956 (carbonyls of metals of Groups VI-B, VII-B and VIII); 3,448,140 (complex compound of a transition metal having atomic number of 21 to 29, 39 to 47 and 71 to 79, inclusive, containing ligands of P, As, or Sb); 3,467,694 (noble metal and a Lewis acid); 3,531,512 (palladium and a Lewis acid); 3,644,462 (noble metal halide and primary, secondary or tertiary amine); 3,895,054 (Se, S or Te); 3,956,360 (Se, S or Te); 3,993,685 (tertiary amine and a platinium group metal or compound thereof); 4,052,437 (rhodium oxide); and 4,080,365 (Se+base+aromatic amino or urea promoter); (2) British Pat. Nos. 1,089,132 (metal carbonyls of Groups VI-B, VII-B and VIII and multivalent metals or their salts); 1,469,222 (palladium group metal halide and a nitrogen-containing heterocyclic compound); and 1,485,108 (Se); and (3) French Patent of Addition No. 2,008,365, as cited in 73 Chem. Abs. 66 302p (1970) (palladium, Al.sub.2 O.sub.3 or Fe.sub.2 O.sub.3).
While the above processes allow direct formation of carbamates from nitro-compounds, a one-step process for converting amines to carbamates would also be useful.
Direct carbonylation of aromatic amines to carbamates in significant yields has not heretofore been achieved. Indeed, it was long believed that carbonylation of such amines only yields ureas or formamides. Thus, Hagelloch, Ber., vol. 83, 258 (1950) reacted aniline and COS in ethanol to form low yields (1-3%) of 1,3-diphenyl urea, and German Patent 863,800 (1953) converted aniline to high yields of urea and/or N-substituted formamides with CO in the presence of nickel iodide, powder nickel or cobalt (activated with MgO or SiO.sub.2) as catalyst.
U.S. Pat. No. 3,099,689 obtained formamides by reaction of aniline with CO in the presence of organometallic compounds of metals of Groups IVB, VB, VIB, VIIB or VIII of the Periodic Table.
U.S. Pat. No. 4,052,454 formed unsymmetrical ureas by the reaction of nitrogeneous organic compounds with aniline, carbon monoxide and sulfur or selenium and certain bases, and British Pat. No. 1,275,702 produced ureas from primary or secondary mono- or diamines by reaction with CO in the presence of Se. N. Sonoda et al., J. Amer. Chem. Soc., vol. 92 (23), p. 6344 (1971) obtained very high yields of urea from ammonia or aliphatic amines, CO and O.sub.2 in the presence of Se, and K. Kondo et al., J. Chem. Soc. Chem. Comm., 307 (1972) obtained stiochiometric yields of urea by carbonylation of aromatic amines with CO, O.sub.2 and Se, employing a strongly basic tertiary amine, such as triethyl amine, as co-catalyst.
R. A. Franz et al., 26 J. Orq. Chem. 3309 (1961) found that tertiary aliphatic amines, KOH and CaO or MgO in methanol were urea catalysts in the reaction of aromatic amines with CO and S. U.S. Pat. No. 2,877,268 disclosed obtention of "excellent yields" of urea by use of alkaline catalysts with a dissociation constant of greater than 1.times.10.sup.-10 : tertiary alkyl amines of 1 to 18 carbon atoms, quaternary ammonium hydroxides, alkaline earth metal and alkali metal hydroxides, alkaline and alkali metal salts (such as sodium oleate), MgO (in methanol), Ca (in methanol) and certain substituted aryl and aralkyl amines. Similarly, diuredides were obtained in Canadian Pat. No. 634,690 by reacting aromatic diamines with CO, S and certain aliphatic or aromatic secondary amines in methanol.
Thio-derivatives of amines have also been produced by carbonylations. U.S. Pat. No. 3,636,104 formed N, N'-diaryl thioureas by reacting aniline with CS.sub.2 in pyridine or alcohol with the addition of S or H.sub.2 O. Alkylamine salts of N-alkyl thiocarbamic acid were prepared in U.S. Pat. No. 2,655,534 by reacting COS and a primary or secondary aliphatic amine. U.S. Pat. Nos. 3,392,197 and 3,539,587 prepared substituted thioureas and monothiocarbamates from primary and secondary amines employing CO and sulfur or sulfur compounds. Thiocarbamates have also been prepared by reaction of amines and disulfides in equimolecular ratio with carbon monoxide in the presence of selenium catalysts and triethylamine. See P. Koch, Tetrahedron Letters No. 25, pp. 2087-2088 (1975); West German Patent Publication No. 2,617,917, 86 Chem. Abs. 43426 m (1977).
In attempting to carbonylate amines to a carbamate product, F. Baiocchi, et al., 21 J. Org. Chem. 1546 (1956) prepared methyl-N-phenyl carbamate in low yield (27-30%, based on aniline charged) by reacting aniline and COS in methanol employing either zinc peroxide, di-tert-butyl peroxide or O.sub.2 to induce the reaction. Magnesium peroxide was found to be not operative, and other peroxides (H.sub.2 O.sub.2 in H.sub.2 O and cumen hydroperoxide in methanol, with and without sodium methoxide), yielded very large amounts of 1, 3-diphenyl urea. Netherlands Pat. No. 94,613 converted aliphatic primary and secondary amines to urethanes by reaction with CO in the presence of alcohols and stoichiometric amounts of certain cupric compounds, which were reduced to the cuprous state, and required reoxidation, as by O.sub.2, to regenerate the cupric reactant.
R. A. Franz et al., 28 J. Org. Chem. 585 (1963) also obtained urethanes from aniline, COS and methanol in the presence of triethyl amine, but could not achieve urethane yields greater than about 13.5%. Even using a carefully controlled, multi-step process, urethane yields greater than 25% were not obtained by Franz et al. under any combination of experimental conditions. Stoichiometric reaction of certain metal acetates (Hg.sup.+2, Tl.sup.+3 and Cu.sup.+2) in T. Saegusa, et al., Tetrahedron Letters No. 42, pp. 4123-4126 (1967) with piperidine, CO and CH.sub.3 OH did not greatly improve urethane yields. Use of the metal acetates of Ag.sup.+1, Cd.sup.+2 and Zn.sup.+2 gave only trace product, even after 98 hours of reaction.
Higher urethane yields have been provided by U.S. Pat. Nos. 3,384,655 (issued in 1968 to Anderson et al.) and 3,629,311 (issued in 1971 to Anderson et al.) and K. Kondo, et al., Chem. Letters, pp. 373-374 (Chem. Soc. Japan 1972). In the Anderson et al. process, a secondary (or a mixture of secondary and teritary) amine is first reacted with COS and an alcohol to form an adduct containing the urea, COS and alcohol, followed by a low temperature oxidation of the adduct with O.sub.2, optionally in the presence of soluble Fe, Ni, Co, Cu, Hg, Pd, Pt or Au halide, sulfate or nitrate promoters, to form the desired urethane, elemental sulfur and water. Kondo et al. reacted a primary amine, CO, Se and methanol in the presence of triethylamine, followed by oxidation with O.sub.2 of the foregoing mixtures, to yield the urethane, and to form a Se precipitate and water. However, the required use of O.sub.2 (or peroxides as in the Baiocchi process) is industrially severely disadvantageous due to the ease with which aniline is oxidized to a wide variety of by-products and due to the obvious explosive hazards associated with mixtures of oxygen, carbon monoxide and alcohol. The explosive hazards require careful attention to temperature controls and use of expensive processing equipment. A further disadvantage to the use of oxygen is the by-product water which is formed and which then reacts with the carbamate to form a urea. To avoid the urea problem, water absorbing agents must be added and additional care must be taken to use anhydrous reactants to avoid further urea being formed. Both of these precautions require added processing expense.